DESCRIPTION: (Adapted from the Applicant's Abstract.) The objectives of the proposed research are to measure the physiological responses of neurons in the vestibular nuclear complex that receive primary inputs from afferent neurons associated with the semicircular canals and the otolith organs and higher order inputs, and to describe the morphological characteristics of both vestibular afferents and their target neurons in the medial and lateral vestibular nuclei. Natural and electrical stimulation of the labyrinth will be used in acute preparations (decerebrate gerbils and guinea pigs) and alert behaving guinea pigs. Vestibular stimulation in the form of purely angular or linear head movements in the vertical and horizontal head planes will be used to test spatial and temporal response properties. The use of translation motion as a natural stimulus will allow evaluation of linear acceleration responses of central vestibular neurons without the confounding effects of co-acting angular acceleration of the head that occurs with time-varying head tilts. Comparisons between data from decerebrate and alert preparations will provide an assessment of the influence of non-vestibular inputs on the response properties of VNC neurons. Further characterizations of responsive neurons will test their relationships to vestibulo-ocular and vestibulo-spinal mechanisms by antidromic identification techniques (acute preparations) and analyses of temporal relationships between neuronal activity and the amplitude, direction or duration of oculomotor or neck muscle responses (alert preparations). The morphological studies are designed to compare the functional properties of vestibular afferent and vestibular nuclear neurons with the geometry of their axonal arborizations and connections. Physiologically characterized neurons will be intracellularly injected with RHP or biocytin to label axonal processes and cell bodies. In double-labeling experiments, physiologically identified utricular and horizontal canal-related afferents will be injected with separate tracers; concurrently, populations of vestibulospinal or vestibulo-ocular neurons will be retrogradely labeled with the colloidal-gold complex, WGAapoHRP-Au, to examine the relationship between classes or primary afferents and second order cells of origins of the vestibular projection pathways. Quantitative assessment of these data will be made using three-dimensional reconstruction techniques and morphometry.